1. Field of the Invention
This invention relates to a zoom lens apparatus with auto-focusing and particularly to a zoom lens apparatus with auto-focusing used for a video camera.
2. Description of the Prior Art
A zoom lens apparatus with auto-focusing is used for taking picture images through a zoom lens system with auto-focusing, which comprises a zoom lens system and a camera body having an imaging device, a drive circuit for zooming the zoom lens system in response to a manipulation switch and moving the focus lens and an auto-focusing circuit for keeping projected optical images on the imaging device in focus by changing the position of a focus lens of the zoom lens system. Such zoom lens apparatus can be used in various types of video cameras. There are two types of zoom lens systems used for such apparatus, namely, front-lens and rear-lens focusing types. In the front-lens focusing type of a zoom lens system, the focus lens is provided on the side of an object with respect to a zoom lens and in the rear-lens focusing type, a focus lens is provided between a zoom lens and the imaging device.
A zoom lens apparatus with auto-focusing using either type of the zoom lens system is operated such that the zoom lens is zoomed by changing the position of the zoom lens in response to a manipulation switch and then, a focus lens is positioned for obtaining just-focus picture images in accordance with a focus signal detected from the imaging device. Position relation between the zoom lens and the focus lens is predetermined with respect to an object distance, i.e., a distance between an object and the zoom lens system, shown by characteristic curves in FIGS. 5 and 6.
The conventional rear-lens focusing type, such as disclosed in the European laid-open patent application No. 260960 filed by the applicant of the present invention, contains a drawback in that a reproduced picture image detected by the imaging device of the apparatus becomes out of focus when the zoom lens system is zoomed from wide-angle to telephoto positions. The reasons are as follows:
If the zoom lens positions at a telephoto position, the auto-focusing circuit detects that the object distance is of characteristic curve U4 because depth of field d1 crosses only the characteristic curve U4, as shown in FIG. 5. However, when the zoom lens position is changed from a wide-angle to telephoto positions, the firstly detected object distance becomes not allowable, because the auto-focusing circuit may detect that the object distance is of characteristic curve U6 at a wide-angle position because depth of field d2 of wide-angle position is approximately the same as depth field d1 of telephoto position and thus, the depth of field d2 crosses the characteristic curves U1 to U4.
Similarly, in the front-lens focusing type, there is also the drawback that a reproduced picture image detected by the imaging device of the apparatus becomes defocused when the zoom lens system is zoomed from wide-angle to telephoto positions. This is because the depth of field d3 at a telephoto position crosses only the characteristic curve U4 but depth of field d4 at a wide-range position crosses characteristic curves from U1 to U7. Therefore, the auto-focusing circuit may detect the distance from the object to be of a characteristic curve other than U4 at a wide-angle position. Therefore, a reproduced picture image detected by the imaging device of the apparatus becomes defocused when the zoom lens system is zoomed from wide-angle to telephoto positions.